JP2002118318A - Molecular fluorine laser system and method for controlling bandwidth thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a more compact, economical fluorine module laser system which emits a beam around 157 nm having a power and a narrow band width necessary for application processing. SOLUTION: The molecular fluorine (F2) laser system comprises a seed oscillator and a power amplifier 14. The seed oscillator includes a laser tube including multiple electrodes therein which are connected to a discharge circuit. The laser tube is a part of an optical resonator for generating a laser beam including the first line of multiple characteristic emission lines around 157 nm. The laser tube is filled with a gas mixture including molecular fluorine and a buffer gas. A low pressure seed radiation generating gas lamp is alternatively used. The gas mixture is at a pressure below that which results in the generation of a laser emission including the first line around 157 nm having a natural line width of less than 0.5 pm. The power amplifier 14 amplifies the power of the beam emitted by the seed oscillator to a desired power for application processing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molecular fluorine laser that emits light at about 157 nm, and more particularly, to an amplifier for increasing the energy of an emitted laser beam to a desired power for an application process. It relates to a very narrow characteristic bandwidth and low spectral purity emission molecular fluorine laser oscillator.

[0002]

_2. Description of the Related Art Ultra-narrow bandwidth emission uses an F ₂ laser to produce a refractive optics projection lens.
ens) in order to be usable in 157 nm lithography. According to where recent survey showed, when the F ₂ laser of normal operating conditions, the natural bandwidth of the emission (natural bandwidth) is about 0.60 ± 0.10pm. Also, due to the special resonator design including sensitive optical elements, linenarrowing can be reduced to about ≤0.15.
pm is possible. However, using such an optical narrowing resonator, the output energy drops significantly to ≦ 1 mJ. Desired output energy, for example, about 10 m
One solution to obtain J is to use an oscillator-amplifier design (see U.S. patent application Ser.
599, 130).

The advantageous oscillator-amplifier design described in the '130 patent application provides a very narrow bandwidth laser beam at the desired energy for application processes such as photolithography. The use of the design described in the '130 patent application also requires two laser systems in parallel, namely the synchronization of the oscillator and the amplifier, which is expensive, takes up a lot of space, and in some situations. May lack reliability. Preliminary experiments have shown that the oscillations described in the '130 patent application-
Amplifier stage has 157 advantages for application process
nm laser beam can be provided.

[0004]

However, the above design also, in some circumstances, may be catadioptric (catalytic).
dioptric lens) as compared to systems using the release of natural bandwidth of F ₂ laser for use with an imaging system having a design to be an economically disadvantageous solution in terms of competitiveness.

Accordingly, it is an object of the present invention to provide a low bandwidth,
It is to provide a more compact and economical molecular fluorine laser system that emits a beam of about 157 nm with sufficient power for the application process.

[0006]

In view of the above object,
Molecular fluorine (F ₂₎ laser system that includes a seed oscillator and (seed oscillator) and a power amplifier is provided. Inside the seed oscillator is included a laser tube that includes a number of electrodes connected to a discharge circuit. Seed radiation
radiation) may alternatively be provided by an excimer lamp, which is preferably maintained at a low pressure.
The laser tube has a multi-characteristic emission line (multip
part of an optical resonator that generates a laser beam that includes the first emission line of the le characteristic emission line. The laser tube is filled with a gas mixture containing molecular fluorine and a buffer gas. The gaseous mixture is 0.1 mm without additional line narrowing optics to narrow the first bright line.
A pressure lower than the pressure that results in the generation of a laser emission that includes a first emission line of about 157 nm having a natural line width of less than 5 pm. The power amplifier increases the power of the beam emitted by the seed oscillator to the desired power for the application process.

[0007] Also, a discharge tube filled with a gas mixture containing molecular fluorine and a buffer gas, a number of electrodes in a discharge chamber connected to a discharge circuit for applying a voltage to the gas mixture; A resonator for generating a laser beam including a first one of the multiple characteristic emission lines;
A molecular fluorine laser system is also provided that includes a power amplifier that increases the power of the beam generated by the resonator to the desired power for the application process. The above gas mixture allows the laser beam to have a line width of less than 0.5 pm, even though the resonator does not include additional line narrowing optical components that further narrow the line width of the first bright line. Having a sufficiently low total pressure to include a first emission line of about 157 nm.

In addition, a discharge tube filled with a gas mixture containing molecular fluorine and a buffer gas, a number of electrodes in a discharge chamber connected to a discharge circuit for applying a voltage to the gas mixture, and about 157 nm. A molecular fluorine laser including a resonator for generating a laser beam including a first one of the multi-characteristic emission lines, and a power amplifier for increasing the power of the beam generated by the resonator to a desired power for an application process. A system is provided. The resonator includes at least one line narrowing optical component for narrowing the line width of the first bright line of about 157 nm. The gas mixture has a sufficiently low total pressure that the narrowed first bright line has a line width of less than 0.2 pm.

Also, a discharge tube filled with a gas mixture containing molecular fluorine and a buffer gas, a number of electrodes in a discharge chamber connected to a discharge circuit for applying a voltage to the gas mixture, about 157 nm. A resonator for generating a laser beam including a first emission line of the multi-characteristic emission line; a gas processing unit coupled to the discharge tube for flowing gas between the discharge tube and the gas processing unit; and a gas mixture. A processor for controlling the flow of gas between the gas processing unit and the discharge vessel to control one or more parameters associated with the laser and the power of the beam generated by the resonator to a desired value for the application process. A molecular fluorine laser system is provided that includes a power amplifier that increases power. The gas mixture has a first beam of about 157 nm with a line width of less than 0.5 pm.
It has a sufficiently low total pressure to include the bright lines.

Also, a discharge tube filled with a gas mixture containing molecular fluorine and a buffer gas, a number of electrodes in a discharge chamber connected to a discharge circuit for applying a voltage to the gas mixture, about 157 nm. A resonator for generating a laser beam including a first one of the multiple characteristic emission lines;
A gas processing unit coupled to the discharge tube for flowing gas between the discharge tube and the gas processing unit; and a gas processing unit and the discharge tube for controlling one or more parameters associated with the gas mixture. And a processor for controlling the flow of gas there between. The gas mixture has a sufficiently low total pressure that the laser beam contains a first emission line of about 157 nm with a line width of less than 0.5 pm.

Also, operating the laser system, monitoring the bandwidth of the output beam of the laser system, and providing the output beam with a multi-characteristic emission line of about 157 nm having a linewidth of less than 0.5 pm. Controlling the gas mixture pressure in the laser tube of the laser system to a predetermined pressure low enough to include a single emission line; and increasing the power of the beam generated by the resonator to the desired power for the application process Amplifying the output beam to cause the excimer or molecular fluorine laser system to control the bandwidth.

In addition, operating the laser system, monitoring the bandwidth of the output beam of the laser system, and providing the output beam with a multi-characteristic emission line of about 157 nm having a linewidth of less than 0.5 pm. Controlling the pressure of the gas mixture in the laser tube of the laser system to a predetermined pressure sufficiently low to include a single emission line. You.

[0013] Also, operating the laser system includes: providing the laser beam at a predetermined pressure sufficiently low that the output beam includes a first one of a multi-characteristic emission line of about 157 nm having a line width of less than 0.5 pm. Controlling the pressure of the gas mixture in the laser tube of the system.

There is also the step of operating the laser system and the radiation emitted by the lamp.
n) monitoring the bandwidth;
The gas lamp of the laser system is brought to a sufficiently low predetermined pressure so as to contain bright lines having a line width of less than 5 pm.
mp) controlling the pressure of the gas mixture during, and amplifying the radiation emitted by the gas lamp to increase the power of the beam generated by the lamp to the desired power for the application process. A method is provided for controlling the bandwidth of an excimer or molecular fluorine laser beam generation system that includes a low pressure gas lamp and an optical amplifier.

Furthermore, a seed radiation generating excimer or a seed radiation generat filled with a gas mixture containing at least molecular fluorine.
Excimer or molecular fluorine (F ₂ ), including ingexcimer or molecular fluorine gas lamp) and power amplifier
A laser system is provided. The gas mixture is 0.5
A pressure lower than the pressure that results in the generation of seed radiation emission having a natural linewidth of less than pm. The power amplifier increases the power of the radiation emitted by the seed radiation generating gas lamp to the desired power for the application process.

There is also provided a molecular fluorine (F ₂ ) laser system including a seed radiation generating molecular fluorine gas lamp and a power amplifier filled with a gas mixture containing at least molecular fluorine. The gas mixture contains about 1 luminous line having a natural line width of less than 0.5 pm.
A pressure lower than the pressure that results in the generation of a 57 nm seed radiation emission. The power amplifier increases the power of the bright line to the desired power for the application process.

[0017]

DETAILED DESCRIPTION OF THE INVENTION (Incorporated by reference) The following is a summary of the above prior art and invention as a disclosure of alternative embodiments of the elements or features of the preferred embodiment that are not described in detail below except as set forth herein. In addition to the references cited in the section of the task to be solved or the means for solving the problem itself, in the cited list of references incorporated by reference into the following detailed description of the preferred embodiments of the present application: is there. One or a combination of two or more of these references may be considered to obtain variations on the preferred embodiment described in the following detailed description. Still other patents, patent applications and non-patent references are cited in the description of this document, and are incorporated by reference into the description of the preferred embodiments with the same effects as described with respect to the following references.

Nos. 09 / 453,670 and 09/447, US patent applications Ser. Nos. 09 / 453,670, assigned to the same assignee as the present application.
No. 882, No. 09 / 317,695, No. 09/51
No. 2,417, No. 09 / 599,130, No. 09/5
No. 98,552, No. 09 / 695,246, No. 09 /
No. 712,877, No. 09 / 574,921, No. 09
/ 738,849 and U.S. Patent No. 6,154,47.
No. 0, No. 6,157,662, No. 6,219,368
No. 5,150,370, No. 5,596,596
No. 5,642,374, No. 5,559,816
No. 5,852,627, 6,005,880
No., and a 5,901,163, and all patents and patent applications and literature references other than patents mentioned in the description of the prior art and the specification of the present application, and K.Vogler, "Advanced F ₂ -Laser
for Microlithography: Recent microlithography for the F ₂ - laser ", SPIE 25th Annual International Sy
mposium on Microlithography, Santa Clara, 2000
28-March 3 Papers p. 1515 and European Patent EP 0 472 727 B1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As will be appreciated in the present application, the natural bandwidth of F ₂ laser emission will depend sharply on the overall gas pressure in the laser tube (see US patent application Ser. No. 09/883, assigned to the same assignee as the application and incorporated herein by reference.
No. 128). According to a preferred embodiment of the present application, the total pressure of the laser gas is, for example, between 3000 mbar (representing normal operating conditions) and 1500 mbar.
Lowering r or even below 1000 mbar results in an advantageous reduction in the natural bandwidth of the emission of the F ₂ laser. In an alternative to many of the preferred embodiments described in this application, a lamp preferably containing low pressure molecular fluorine is used to generate a narrow bandwidth seed radiation. This alternative embodiment may be used to generate a narrow band radiation of about 157 nm, and by using a lamp having a gas mixture containing an appropriate amount of argon or krypton, respectively, to about 193 nm or about 193 nm, respectively. Sometimes used to provide 248 nm narrow band radiation. The natural or characteristic bandwidth of the approximately 157 nm spectral emission line of the F ₂ laser is advantageously in this way 0.4 pm, or even 0.2 pm or 0.1 pm.
m. These bandwidths are 0.4p
exposure radiation of m or less
Two materials for color correction (chromatic correction) (e.g., CaF ₂ and BaF _2), or 0.2pm
Or less than the bandwidth used in refractive optics lens designs that include either only a single optical material (eg, CaF ₂ ) for irradiating radiation.

This pressure drop also involves a reduction in output energy, which may be a tenth or more drop in energy. Thus, as will be further appreciated in the present application, an amplifier may be used to generate a beam having sufficient energy. As will be further recognized in the present application, expensive and sometimes unreliable optical components typically used to produce very narrow bandwidths can be omitted, and line narrowing optics can be eliminated. the seed radiation emitted from the F ₂ laser tube of low gas pressure without components in the second laser tube, can be amplified to a desired level for the application in 157nm lithography. In the alternative embodiment described above, where a low pressure lamp is used to provide the seed radiation to be amplified by the amplifier, the amplifier will in fact also reduce the seed radiation emitted by the lamp to 157 nm,
Amplify to the desired power at 193 nm or 248 nm for lithographic processing. An important advantage is the is used to generate the F ₂ laser gas emission very narrow linewidth serve seed radiation for large amplification stage, a very simple, compact, inexpensive It is a low pressure discharge tube, or just a lamp. Further, if desired, such linewidth narrowing optical element (s) may be used to further reduce linewidth. The inventor's own measurements showed that about 100 μJ
Seed energy is sufficient to effectively amplify up to 10 mJ in the amplification stage. Even a normally large size amplifier laser system with a very small low pressure seeding discharge is advantageously less expensive than a conventional large single stage oscillator itself.

According to one embodiment of the present application, it is an object of the present invention to provide a method for controlling a line-narrowed oscillator, which is typically one-sided.
oscillator), with sufficient power without large and costly efforts requiring two large lasers, the other acting as a power amplifier, ≦ 0.4
To produce a very narrow linewidth F ₂ laser emission, such as pm or ≦ 0.2 pm. By applying this preferred embodiment, at a lower than normal pressure that serves to produce the desired very narrow bandwidth without having additional line narrowing optical components in the laser cavity, laser dimension) powerful laser amplifier is combined with small and inexpensive laser discharge tube for accommodating the same F ₂ laser gas mixture with a large laser described above. Furthermore, this small and simple discharge tube can advantageously be integrated in the usual laser cabinet of the amplifier. Therefore, basically, a laser system that generates the beneficial very narrow F ₂ lasing with sufficient energy is achieved with only one system. Such a system can advantageously compete with conventional single stage oscillator designs for catadioptric optical projection systems. According to an alternative embodiment, line narrowed oscillator having a reduced gas mixture pressure is used to further narrow linewidth line width of 157nm emission line F ₂ lasing.

FIG. 1 shows a typical energy of between about 0.1 mJ and about 0.1 mJ, and even more preferably about 10 mJ.
Emits a 157 nm beam containing a selected bright line with a natural linewidth of 6 pm ± 0.1 pm and 2500 mbar
1 schematically illustrates a molecular fluorine laser having a higher gas mixture pressure. The oscillator shown in FIG. 1 can operate at reduced gas mixture pressure to produce a 157 nm beam with reduced linewidth according to the preferred embodiment.

Referring to FIG. 1, a single step F ₂ emitting a natural bandwidth at a normal energy level, eg, 10 mJ.
An outline of the laser (a variant of the oscillator only) is shown. The illustrated F ₂ laser includes a laser chamber 1 containing an F ₂ laser gas mixture containing molecular fluorine and a buffer gas. The gas mixture is about 2500-300
It may be at a normal pressure of 0 mbar and may operate at reduced pressure according to the preferred embodiment of the present application. FIG.
In addition, the F ₂ laser shown in FIG.
dispersion arrangement, for example, includes a bright line selection unit 2 containing a dispersing prism,
Or an interferometer device such as an etalon, or US patent application Ser. Nos. 09 / 715,803 and / or 60 / 280,398, assigned to the same assignee as the present application and incorporated herein by reference. Alternative or additional optical devices may be included, such as devices with non-parallel plates as described in. Bright line selection unit 2 is about 1
May be omitted for 57 nm multiple emission emission and, if used, are preferably about 157.63 n
It is configured to select m main emission lines. The F ₂ laser, schematically shown in FIG. 1, further includes an output coupling mirror (output
A coupler mirror 3, a laser discharge circuit and a solid state pulser 4, and a high-reflection mirror 5 are included, but partially reflected output couplers.
An alternative to the utput coupler 3 and the HR mirror 5 may be provided as discussed below with reference to FIG.

FIG. 2 shows that the pressure of the gas mixture is 2500-3.
157n including selected narrowed emission lines having a bandwidth of less than 0.5pm, such as about 0.2pm, at a typical energy of about 1mJ, even at 000mbar.
Although a schematic of a molecular fluorine laser emitting an m-beam is illustrated, the pressure of the gas mixture can be reduced according to a preferred embodiment of the present application. Referring to FIG. 2, stage F ₂ laser (only oscillator) is shown. The F ₂ laser, schematically illustrated in FIG. 2, preferably includes the same or similar laser chamber 1, as described above with respect to FIG.
A discharge circuit and solid pulser module 4 and an output combiner 3 are included (see discussion below with reference to FIG. 8 for more details of a suitable system). In the optical resonator,
Includes advanced optical designs that include specialized line narrowing modules 12 that include complex and expensive optical components as compared to the design of FIG. Stability may be somewhat lower at 157 nm). The module 12 may be configured for single line emission line selection in addition to the selected line narrowing, and may also narrow one (or more) of the approximately 157 nm emission lines. It may be configured just for you. Many components, including prisms, inserted into the resonator beam path of the system of FIG. 2 produce greater losses than those of the simple design shown in FIG. That is, the total output energy is reduced by each element used for improving the narrowing of the line width.

FIG. 3 shows an energy between 1 and 15 mJ, particularly preferably about 10 mJ, such as about 0.2 pm.
1 schematically illustrates a molecular fluorine laser including an oscillation-amplifier configuration that produces a 157 nm beam including a selected narrowed emission line having a bandwidth of less than 0.5 pm. The device of FIG. 3 is the same or similar to that described with reference to FIG. 2, but includes a power amplifier 14 that increases the energy of the beam emitted by the oscillator.
Is included.

Referring to FIG. 3, F ₂ laser system oscillation - schematic of an amplifier arrangement (two-stage F ₂ laser system) is shown. The oscillator shown in FIG.
A laser chamber 1, a discharge circuit including a solid-state pulsar module 4, and an output coupler 3,
And a line narrowing module 12. Low-level output energy of the oscillator including a line narrowing module 12, in a subsequent F ₂ amplifier comprising an amplification chamber 14 which is separated from the discharge circuit _{16 (following up F 2 amplifier)} , for example between 1～15MJ, Particularly preferably, it is amplified to the desired level of about 10 mJ, which is usual for lithographic applications. The gas mixture in the oscillation chamber 1 and the amplification chamber 14 may be operated at normal laser gas pressures, for example P = 2,500-3000 mbar, and the gas pressure is reduced, especially in the oscillation chamber 1. Sometimes it is driven by. Both individually designed and optimized discharge circuits 4 and 16 (both preferably comprising a solid-state pulsar module) are precisely synchronized by the synchronization module 7, for example within ≦ 2 μs (each one U.S. Pat. No. 6,00,000 assigned to the same assignee as the application and incorporated herein by reference.
No. 5,880 and US patent application Ser. No. 60 / 204,095.
No.). Both unreliable synchronization and strong spontaneous emission of the oscillator stage can result in strong background radiation of the amplified narrowband signal as illustrated in FIG. That is, the entire system is very expensive because it contains almost two complete laser systems, produces a very large, narrow linewidth output with somewhat poorer spectral purity. Overall, the system schematically illustrated in FIG. 3 can achieve the desired power for lithographic applications, for example, a narrow linewidth at 10 mJ, but adds natural bandwidth to, for example, catadioptric lens systems. The system shown in FIG. 1 may not be competitive.

FIG. 4 shows a 157 nm beam containing an emission line having a natural linewidth of less than 0.5 pm, such as about 0.2 pm, at an energy of between 1 and 15 mJ, especially for lithographic applications, preferably about 10 mJ. 1 schematically illustrates a molecular fluorine laser including a low gas pressure seeded oscillator-amplifier configuration for generating a beam. Referring to FIG. 4, a quasi-single stage amplifier according to a preferred embodiment is shown.
stage amplifier arrangement) is shown. Only one laser module with a small seed tube 18 having a low gas mixture pressure is used. For example, less than 2000 mbar, preferably 1000-150
A low pressure seed tube with a pressure of less than 0 mbar is used as a gain laser module (amplifier).
The same laser gas mixture as, for example, F ₂ (Ne or He
5%): Ne or He = (1-2)%: (99-9)
8)%, but a much lower pressure laser gas mixture having a very narrow linewidth of the emission line of about 157.6309 nm, for example less than 0.5 pm and preferably less than 0.2 pm To accommodate. This radiation is preferably P = 2500-4000 mbar (same gas mixture)
In a conventional pressure amplifier module comprising an amplification chamber 14 having a pressure circuit and a discharge circuit 20, up to a level of 1 to 15 mJ, particularly preferably about 1 to 15 mJ.
It is amplified to a level of 0 mJ. Since very low pressure seed radiation is only about 100 μJ, amplification factors of 100 or more for full amplification are advantageously easily achievable with the preferred embodiment system. The low-pressure discharge vessel or module is advantageously small. Thus, the background radiation level entering the amplifier from this small tube 18 is greatly reduced from that produced by the system of FIG. The seed tube 18 is pre-discharged, for example, by a separate part 22 of the amplifier discharge module 20 in a manner similar to the pre-ionization of a conventional excimer discharge tube. This system can also operate without the synchronization module 7 of the device of FIG. The HR mirror 24 provides backward laser emission.

FIGS. 5A and 5B are qualitative examples of the bandwidth and spectral purity of the beam emitted by the molecular fluorine laser of FIGS. 3 and 4, respectively. Referring to FIGS. 5 (a) and 5 (b), the spectral purity of both amplifier configurations shown in FIGS. 3 and 4, respectively, is shown. As illustrated in FIG. 5 (a), due to the high pressure and long background length of the long gain length (a laser module of normal size), the system of FIG. A strong ASE background occurs in the amplified output of

In contrast, FIG. 5 (b) illustrates the output of the system of FIG. The small, low-pressure sub-module, which acts as a seed source for the downstream amplifier, provides a substantial background ASE that can be amplified with narrow bandwidth emission in high-pressure amplifiers.
It does not emit radiation. Thus, the spectral purity of the narrow-bandwidth emission mechanism schematically illustrated in FIG. 4 is advantageous for lithographic applications because the ASE level is negligible.

FIG. 6A shows that the F ₂ laser is 1600 mb.
Quantitatively illustrates the molecular fluorine laser emission spectrum when operating at a reduced gas pressure of ar and having a bandwidth of less than about 0.47 pm without deconvolution by the spectrometer apparatus function. I do. In contrast, FIG. 6 (b), F ₂ laser quantitatively illustrate the molecular fluorine laser emission spectrum when having a bandwidth of approximately 1pm operate at gas pressure 2500～3000Mbar.

FIG. 7 illustrates the dependence of the bandwidth of the F ₂ laser emission on the total pressure of the laser gas mixture in the laser tube, with a nearly constant fluorine-buffer gas distribution (partitio).
Figure 4 shows the reduction in bandwidth (i.e., FWHM of the main emission line at about 157.63 nm) with decreasing total pressure of the laser gas mixture in n). F ₂ laser gas mixture used in the measurement of the graph of Figure 7, as illustrated, about 4000-15
F ₂ (5% in Ne) for a gas pressure of 00 mbar:
He = (1-2%: 99-98%). About 4
For a gas pressure of 000 mbar, the bandwidth is about 1.2 p
m is observed. At a gas pressure of about 3000 mbar, the bandwidth is observed to be about 0.8 pm. At a gas pressure of about 2000 mbar, the bandwidth is observed to be about 0.6 pm. About 1500
For a bath pressure of mbar, the bandwidth is observed to be about 0.4 pm. At a gas pressure of about 1000 mbar, the bandwidth is observed to be about 0.3 pm. At a gas pressure of about 800 mbar, the bandwidth is observed to be about 0.2 pm. At a gas pressure of about 600 mbar, the bandwidth is observed to be about 0.15 pm. It is observed that the relationship between bandwidth and total pressure of the laser gas is very linear. (Overview of Overall Laser System) FIG. 8 illustrates an overview of an overall molecular fluorine laser system according to the preferred embodiment. Although the seed oscillator shown in FIG. 4 is generally simpler than that described below, the selected configuration may include some details of the system described below,
On the other hand, the systems of FIGS. 1-3 are more generally related to the illustrated devices. Referring to FIG. 8, an outline of an excimer or molecular fluorine laser system is shown according to a preferred embodiment. A preferred gas discharge system is vacuum ultraviolet (VU)
V: A VUV laser system, such as a molecular fluorine (F ₂ ) laser system used in lithography systems. For example, alternative configurations of laser systems used in other industrial applications such as TFT annealing, photoablation and / or micromachining may be similar and / or similar to the system shown in FIG. 8 to meet the requirements of that application. It includes configurations that would be understood by those skilled in the art as modified. Alternative DUV or VUV laser systems and component configurations for this purpose are disclosed in US patent application Ser. No. 09 / 317,695, each assigned to the same assignee as the present application and incorporated herein by reference.
No. 09 / 130,277, 09 / 244,55
No. 4, 09/452, 353, 09/512, 4
No. 17, No. 09/599, 130, No. 09/694,
No. 246, No. 09 / 712,877, No. 09/57
No. 4,921, No. 09 / 738,849, No. 09/7
18,809, 09 / 629,256, 09 /
No. 712,367, No. 09 / 771,366, No. 09
No./715,803, No. 09 / 738,849, No. 6
Nos. 0 / 202,564, 60 / 204,095, 09 / 741,465, 09 / 574,921,
No. 09 / 734,459, No. 09 / 741,465
No., 09 / 686,483, 09 / 715,80
No. 3,09,780,124, and U.S. Patent Nos. 6,005,880, 6,061,382, 6,020,723, 5,946,337, 6,6. No. 014,206, No. 6,157,662, No. 6,154,470, No. 6,160,831, No. 6,160,832, No. 5,559,816, No. 4,611, No. 270, 5,761,236, 6,212,214, 6,154,470, and 6,157,662.

The system shown in FIG. 8 generally includes a laser chamber 102 (or a heat exchanger and a gas in the chamber 102 or tube) having a pair of main discharge electrodes 103 connected to a solid pulser module 104. A laser tube including a fan for circulating the mixture), and a gas processing module 106. Since the gas processing module 106 has a valve connection to the laser chamber 102,
In the case of ArF, XeCl and KrF excimer lasers, the halogen, noble and buffer gases, and preferably gas additives, are preferably in a premixed form.
m) (US patent application Ser. No. 09 / 513,025, assigned to the same assignee as the present application, and US Pat. No. 4,977,5).
73 No. See, it is injected or filled into the laser chamber at each are hereby incorporated by reference), in the case of F ₂ laser halogen and buffer gas,
And some gas additives are injected or filled. For high power XeCl lasers, the gas processing module may or may not be present throughout the system. The solid pulser module 104 is powered by a high voltage power supply 108. Thyratron Pulsa
Modules may be used instead. laser·
The chamber 102 includes an optical module 1 forming a resonator.
10 and the optical module 112. The optics module may include only a high reflection resonant reflector in the rear optics module 110 and a partially reflecting outcoupling mirror in the front optics module 112, as is preferred for a high power XeCl laser. The optics modules 110 and 112 are controlled by the optics control module 114 or, alternatively, in particular KrF,
As is preferred when an ArF or F ₂ laser is used for optical lithography, if line narrowing optics are included in one or both of the optical modules 110, 112,
Controlled directly by a computer or processor 116.

The processor 116 for laser control receives various inputs and controls various operating parameters of the system. Diagnostic module 118 is preferably a module 118, such as beam splitter module 122.
Of the separated portion of the main beam 120 via an optical device for deflecting a small portion of the beam in the direction of
One or more parameters such as energy, average energy and / or power, and preferably wavelength are received and measured. Beam 120 is preferably a laser output to an imaging system (not shown), and ultimately to a workpiece (also not shown), especially in lithographic applications, and is output directly to the application process. Sometimes. Laser control computer 116 can communicate with stepper / scanner computer 126, other control units 128, and / or other external systems through interface 124. (Laser Chamber) The laser chamber 102 contains a laser gas mixture and, in addition to a pair of main discharge electrodes 103, one or more preionization electrodes.
ectrode) (not shown). Preferred main electrode 103
Are described in US patent application Ser. No. 09 / 453,670, assigned to the same assignee as the present application for optical lithography applications and incorporated herein by reference, for example, having a narrow discharge width. If not preferred, alternative configurations may be used. Other electrode configurations are disclosed in US Pat. No. 5,729,565, each assigned to the same assignee.
And alternative embodiments are described in U.S. Pat. Nos. 4,691,322, 5,535,233 and 5, all of which are incorporated herein by reference. , 557, 629. Suitable preionization unit was assigned to the same assignee as the respective present patent application U.S. Patent Application Serial No. 09 / 692,265 (particularly suitable KrF, ArF, against F ₂ laser), 09 / 532,276 No. 09 / 247,887, and an alternative embodiment is disclosed in US Pat.
Nos. 337,330, 5,818,865 and 5,
No. 991,324, and all of the above patents and patent applications are incorporated herein by reference. (Solid pulser module) Solid or thyratron
The pulsar module 104 and the high voltage power supply 108 supply the electrical energy in a compressed electrical pulse to the pre-ionization and main electrodes 103 in the laser chamber 102 to provide energy to the gas mixture. The preferred pulsar module and high voltage power components are each assigned to the same assignee as the present application,
U.S. patent applications Ser. Nos. 09 / 640,595, 60 / 198,058, which are incorporated herein by reference.
Nos. 60 / 204,095, 09 / 432,348 and 09 / 390,146, and 60/204,
095, and U.S. Patent Nos. 6,005,880, 6,226,307 and 6,020,723. Other alternative pulsar modules are disclosed in U.S. Pat.
No. 982,800, No. 5,982,795, No. 5,9
Nos. 40,421, 5,914,974, 5,94
9,806, 5,936,988, 6,02
Nos. 8,872, 6,151,346 and 5,72
No. 9,562.

The laser chamber 102 is sealed by a window that is transparent to the wavelength of the emitted laser radiation 120. This window is a Brewster windo
w) and may be aligned at another angle to the optical path of the resonant beam, for example 5 °. One of the windows may serve to out-couple the beam, and may also serve as a highly reflective resonant reflector on the opposite side of the chamber 102 when the beam is out-coupled. (Laser Resonator) The laser resonator surrounding the laser chamber 102 containing the laser gas mixture preferably includes line-narrowing optics for excimer or molecular fluorine lasers having a reduced linewidth. An optical module 110 is included, for example, for optical lithography, where narrowing is undesirable (eg, in the case of TFT annealing) or narrowing is a front optics module. ) 112 or
Or, if a spectral filter external to the resonator is used, or if the narrowing optics is placed in front of the HR mirror to narrow the bandwidth of the output beam, the high It can be replaced by a reflectance mirror or the like. According to a preferred embodiment of the present application, about 15
An optical device for selecting one of the multiple emission lines of 7 nm, for example one or more dispersive prisms or birefringent plates or blocks, may be used, wherein the selected emission lines are narrowed. Additional line narrowing optics that widen may be omitted. To produce selected emission lines with a narrow bandwidth, such as 0.5 pm or less, without using additional line narrowing optics, the total pressure of the gas mixture is preferably lower than in conventional systems, For example, less than 3 bar.

As already discussed, the optical device is either not included, or simply includes only a simple, less lossy optical configuration. Preferably and advantageously, the preferred embodiment of the present invention has no additional line narrowing optics in the laser cavity or λ ₁ ≒ 1
Select the main emission lines of 57.63094Nm, it contains only the bright line selecting optical apparatus for also suppressing any other emission line of approximately 157nm emitted naturally by the F ₂ laser. Thus, in one embodiment, the optical module 110
Is a highly reflective resonator mi
rror), and the optical module 112 has a partially reflective resonator reflector.
r) only. In another embodiment, for example, manufactured from a VUV transparent block having a partially reflective internal surface and having a block or coating of birefringent material, either of which is periodic by interference and / or birefringence, and An output combiner (o) having a transmission spectrum with a maximum at λ ₁ and a minimum at the secondary emission line
by Utcoupler), other about 157 nm (i.e., other than the lambda ₁₎ emission line suppression is performed. In another embodiment, a simple optical device, such as a dispersing prism (s), may be used only for line selection and not for line narrowing of the main line at λ _1. . Other examples of bright line selection are described in U.S. Patent Application No. 09, assigned to the same assignee as the present application and incorporated herein by reference.
Nos./317,695, 09 / 657,396 and 09 / 599,130. Due to the advantageous gas mixture pressure of the seed laser of the preferred embodiment, a narrow band of, for example, less than 0.5 pm without additional line narrowing of the main emission line of λ ₁ using additional optics. The width becomes possible.

The optical module 112 preferably includes a means for outcoupling the beam 120, such as a partially reflecting resonant reflector. Otherwise, beam 120 is outcoupled, such as by an intracavity beam splitter or a partially reflective surface of another optical element, and optical module 112 would then include a highly reflective mirror. The optics control module 114 preferably receives and interprets signals from the processor 116 and initiates a realignment, gas pressure adjustment, or reconfiguration procedure in the modules 110, 112. Optical modules 110 and 1
12 (the above-mentioned '353,' 695, '2
Nos. 77, '554 and' 527). Diagnostic Module After a portion of the output beam 120 passes through the output combiner of the optical module 112, the output portion preferably deflects a portion of the beam toward the diagnostic module 118, or another method. Impinges on a beam splitter module 122 that includes optics to allow a small portion of the beam output coupled to the diagnostic module 118, while the main beam portion 120 outputs the laser system output beam 120. (US patent application Ser. Nos. 09 / 771,013, 09/598, assigned to the same assignee as the present invention).
552, and 09 / 712,877, and U.S. Patent No. 4,611,270. Each of which is incorporated herein by reference.) Suitable optics include a beam splitter or other partially reflective surface optics. The optical device may also include a mirror or beam splitter as the second reflective optical device. 1
More than two beam splitters and / or HR mirrors and / or dichroic mirrors
May be used to direct a portion of the beam to components of the diagnostic module 118. Holographic beam sampler, transmission type diffraction grating, partial transmission type reflection diffraction grating, grism,
Prisms or other refractive, dispersive and / or transmissive optic (s) may also be used to separate small beam portions from main beam 120 for detection at diagnostic module 118. And on the other hand allow the majority of the main beam 120 to reach the application process directly or via an imaging system or otherwise. These or additional optics provide for the red emission from atomic fluorine in the gas mixture from the separated beam before detection.
Used to filter out visible radiation such as

If the output beam 120 is a beam splitter
The reflected beam portion while transmitting through the module may be directed to the diagnostic module 118 or the main beam 120 is reflected while the smaller portion is
18. The portion of the out-coupled beam that persists past the beam splitter module is the output beam 120 of the laser, which in the case of photolithography applications is directed towards industrial or laboratory applications such as imaging systems and workpieces. Propagate.

The diagnostic module 118 preferably includes at least one energy detector. This detector measures the total energy of the beam portion that directly corresponds to the energy of the output beam 120 (US Pat. Nos. 4,611,270 and 6,212,214, which are incorporated herein by reference). reference). Optical attenuator, such as a plate or a coating
An optical configuration, such as, or other optical device is formed on or near the detector or beam splitter module 122 to control the intensity, spectral distribution and / or other parameters of the radiation incident on the detector. (US Patent Application Serial No. 09/17, each assigned to the same assignee as the present application and incorporated herein by reference).
No. 2,805, No. 09 / 741,465, No. 09/7
12,877, 09 / 771,013 and 09
/ 771,366).

One of the other components of the diagnostic module 118
One preferably has a wavelength and / or a wavelength such as a monitor etalon or a grating spectrometer.
Or a bandwidth sensing component (US patent application Ser. No. 09 / 416,344, each assigned to the same assignee as the present application).
No. 09 / 686,483 and 09/791,
431 and U.S. Patent Nos. 4,905,243, 5,
No. 978,391, No. 5,450,207, No. 4,9
No. 26,428, No. 5,748,346, No. 5,02
No. 5,445, No. 6,160,832, No. 6,16
Nos. 0,831 and 5,978,394, all of the above wavelength and / or bandwidth detection and monitoring components are incorporated herein by reference). According to the preferred embodiment herein, the bandwidth is monitored and controlled in a feedback loop including the processor 116 and the gas processing module 106. The total pressure of the gas mixture in the laser tube 102 is controlled to a particular value to produce a beam power at a particular bandwidth.

Other components of the diagnostic module include US patent application Ser. No. 09 / 484,818, each assigned to the same assignee as the present application and incorporated herein by reference.
And a pulse shape detector such as those described in 09 / 418,052, respectively.
ctor) or an ASE detector, which provides gas control and / or output beam energy stabilization, etc., and as described in further detail below, Natural radiation (amplifie
d spontaneous emission (ASE)
Ensure that E remains below a predetermined level. For example, US Patent No. 6,011, assigned to the same assignee as the present application and each patent document being incorporated herein by reference.
There may also be a beam alignment monitor as described in US Pat. No. 4,206 or a beam profile monitor, for example, from US patent application Ser. No. 09 / 780,124. (Beam path enclosure)
In the case of the system and in the case of an ArF laser system, an enclosure 130 encloses the beam path of the beam 120, such as to keep the beam path unaffected by photoabsorbing species. Small enclosures 132 and 134 seal between the chamber 102 and the optical modules 110 and 112, respectively, and an additional enclosure 136 is located between the beam splitter 122 and the diagnostic module 118. Suitable enclosures are described in U.S. patent application Ser. Nos. 09 / 598,552, 09/594, assigned to the same assignee as the present application and incorporated herein by reference.
No. 892, and Ser. No. 09 / 131,580, and U.S. Pat.
No. 19,368, No. 5,559,584, No. 5,22
No. 1,823, No. 5,763,855, No. 5,81
Nos. 1,753 and 4,616,908. (Processor Control) The processor or control computer 116 includes, among other input or output parameters of the laser system and output beam, pulse shape, energy, ASE, energy stability, energy overshoot in the case of burst mode operation, Receive and process some values in wavelength, spectral purity, and / or bandwidth. Processor 116 also includes a line narrowing module.
To tune the wavelength and / or bandwidth or spectral purity, and to control the power supply 108 and the pulser module 104 to preferably control the moving average pulse power or energy, and thus The amount of energy irradiation at each point is stabilized near a desired value. Further, the computer 116 controls the gas processing module 106 that includes gas supply valves connected to various gas sources. Still other functions of the processor 116, such as to provide overshoot control, energy stability control and / or monitor input energy to the discharge, are assigned to the same assignee as the present application and are incorporated herein by reference. See U.S. Patent Application Ser.
No. 561 is described in further detail.

As shown in FIG.
Preferably communicates individually or in combination with the solid or thyratron pulsar module 104 and the HV power supply 108 and communicates with the gas processing module 106, the optics module 110 and / or 112, the diagnostics module 118, and the interface 124. The laser cavity surrounding the laser chamber 102 containing the laser gas mixture includes an optical module 110 including line narrowing optics for a line narrowed excimer or molecular fluorine laser. Is the case where narrowing is not desired, or if narrowing is performed in the front optics module 112 or if a spectral filter external to the resonator is used to narrow the line width of the output beam May be replaced by a high reflectivity mirror or the like in the laser system. Some variations of the line narrowing optics are described in detail below. (Gas Mixture) The laser gas mixture is first filled into the laser chamber 102 in a process referred to herein as "fresh fill". During such a process, the laser tube is exposed to laser gas and contaminant
Is exhausted and refilled with a new gas of ideal composition.
The gas composition for a very stable excimer or molecular fluorine laser according to the preferred embodiment uses helium or neon or a mixture of helium and neon as a buffer gas depending on the particular laser used. Suitable gas compositions are described in U.S. Patent Nos. 4,393,405, 6,157,162 and 4,977,573, each assigned to the same assignee as the present application and incorporated herein by reference. No. and U.S. patent application Ser.
No. 025, No. 09 / 447,882, No. 09/41
8,052, and 09 / 588,561. The concentration of fluorine in the gas mixture is 0.003%
1.00%, preferably about 0.1%. Additional gas additives, such as noble gases or others, may be added to increase energy stability or control overshoot and / or 09 / 513,025, incorporated herein by reference. As an attenuator as described in the patent application,
May be added. That is, when the F ₂ laser, is that the addition of xenon and / or argon is used. The concentration of xenon or argon in the mixture is 0.0
001% to 0.1%. For an ArF laser, the addition of xenon or krypton, also having a concentration of 0.0001% to 0.1%, may be used. Also in the case of KrF laser, 0.0001% ~
Addition of xenon or argon with a concentration of 0.1% may be used. The preferred embodiment of the present application is specifically F
₂ Related to use with lasers, but ArF, K
Several gas replenishment operations with respect to the gas mixture composition of rF and other systems such as XeCl excimer lasers are described, wherein the ideas described in this application are also advantageously incorporated into these systems.

In the case of the F ₂ laser having the above-mentioned structure, the gas composition uses helium, neon, or a mixture of helium and neon as the buffer gas. The concentration of fluorine in the buffer gas is preferably 0.003%
To about 1.0%, preferably about 0.1%. However, if the total pressure is reduced to reduce the bandwidth, the fluorine concentration may be higher than 0.1% despite the total pressure in the tube or the percentage concentration of halogen in the gas mixture, for example, , Between 1 and 7 mbar, more preferably between about 3 and 5 mbar. Laser beam energy stability, burst control,
And / or a small amount of xenon and / or argon to improve the output energy of the laser beam.
Additional oxygen and / or krypton and / or other gases (see the '025 patent application) may be used. The concentration of xenon, argon, oxygen or krypton in the mixture is between 0.0001% and 0.1%.
% And preferably will be well below 0.1%. Several alternative gas configurations, including trace gas additives, are disclosed in US patent application Ser. No. 09 / 513,025 and US Pat. No. 6,157, No. 662.

[0043] Preferably, mixtures of 5% _{F 2 (5% F 2 inNe} with He) in Ne with He as a buffer gas is used, use more or fewer He or Ne it It may be done. The total gas pressure is advantageously between 1500 and 400 to adjust the bandwidth of the laser.
Adjustable between 0 mbar. The partial pressure of the buffer gas is preferably adjusted to adjust the total pressure such that the amount of molecular fluorine in the laser tube does not change from an optimal preselected amount. With decreasing He and / or Ne buffer gas in the gas mixture,
It is shown that the bandwidth is advantageously reduced. That is,
The partial pressure of He and / or Ne in the laser tube can be adjusted to adjust the bandwidth of the laser emission. (Replenishment of gas mixture) For example, noble gas-
Rare gas-halide excimer
In the case of laser, for example, micro-halogen infusion of, for example, 1 to 3 milliliters of a halogen gas mixed with 20 to 60 milliliters of a buffer gas.
halogen injection, total pressure regulation and gas displacement procedures, preferably including vacuum pumps, valve network
k) and one or more gas compartments
This is performed using a gas processing module 106 including
The gas processing module 106 includes a gas container, a tank, and a can (ca).
nister) and / or through a gas line connected to the bottle. Some suitable alternative gas treatment and / or replenishment measures other than those specifically described herein (see below) are described in US Pat. No. 4,977,573, each assigned to the same assignee as the present application. No. 6,21
Nos. 2,214 and 5,396,514 and U.S. Patent Application Nos. 09 / 447,882, 09 / 418,05.
No. 2, No. 09/734, 459, No. 09/513, 0
No. 25, and 09 / 588,561, and U.S. Pat. Nos. 5,978,406, 6,014,398 and 6,028,880, all of which are hereby incorporated by reference. Incorporated in the description of the specification. According to the '025 patent application referred to above, a xenon gas source may be included either inside or outside the laser system.

A total pressure adjustment in the form of a gas release or a reduction of the total pressure in the laser tube 102 may be made. For example, if it is determined after the total pressure adjustment that a component other than the desired partial pressure of the halogen gas exists in the laser tube 102, the gas composition adjustment may be performed following the full pressure adjustment. Total pressure adjustment may be performed after gas replenishment operation,
It may also be done in combination with the adjustment of the drive voltage for the discharge, but the adjustment of the drive voltage is smaller than it would be without pressure adjustment.

A gas displacement procedure may be performed, which is described in Section 0, which is incorporated herein by reference.
As described in the 9 / 73,459 patent application, depending on the amount of gas to be displaced, for example from a few milliliters to 50 liters or more, but less than a fresh charge, , Called a mini or macro gas displacement operation. As an example, the gas processing unit 106 connected to the laser tube 102 either directly or through an additional valve assembly that includes a small compartment to regulate the amount of gas injected (see the '459 patent application) includes % F ₂ : 99% premixed gas A containing other buffer gases such as Ne or He (premix)
For injecting a premixed gas B containing 1% rare gas: 99% buffer gas in the case of a rare-gas halide excimer laser In this case, the premixed gas B is not used in the case of the F ₂ laser.
To add or reduce the total pressure, i.e., possibly with a halogen injection to maintain the halogen concentration,
Separate lines may be used to allow the buffer gas to flow into the laser tube or to allow some of the gas mixture in the tube to be released. That is, by injecting the premixed gas A (and the premixed gas B in the case of a rare gas halide excimer laser) into the tube 102 via the valve assembly, the fluorine concentration in the laser tube 102 is replenished. There is. Next, an amount of gas is released corresponding to the amount injected to maintain the total pressure at the selected level. Additional gas lines for injecting additional gas mixtures and / or
Or a valve may be used. New filling, partial and mini gas replacement and gas injection procedures, for example enhanced micro-halogen injection and normal micro-halogen injection, such as between 1 ml or less and 3-10 ml, and any and all other The gas refill operation is initiated and controlled by the processor 116 which controls the gas processing unit 106 and the valve assembly of the laser tube 102 based on various input information in the feedback loop. Such a gas replenishment procedure may be used in combination with a gas circulation loop and / or window replacement procedure to achieve a laser system with extended inspection intervals for both the gas mixture and the laser tube window.

The halogen concentration in the gas mixture is kept constant during the laser operation by a gas replenishment operation by replenishing the amount of halogen in the laser tube for the preferred molecular fluorine laser in this application. The gas is maintained at the same predetermined ratio as in laser tube 102 after the new fill procedure. Further, as will be appreciated from the above-mentioned '882 patent application, gas injection operations such as μHI are advantageously modified to micro gas displacement procedures, so increasing the energy of the output laser beam reduces the total pressure. Can be compensated. In contrast, or alternatively,
Conventional laser systems will reduce the input drive voltage so that the energy of the output beam is at a predetermined desired energy. In this way, the drive voltage is H
The gas treatment is maintained within a small range around V _opt , while the gas treatment replenishes the gas and averages the pulse rate, such as by controlling the output rate of change or gas flow rate of the gas mixture through the laser tube 102. Operate to maintain energy or energy exposure dose. Advantageously, with the gas treatment described in this application, the laser system achieves average pulse energy control and gas replenishment,
And operate within a very small range near the HV _opt while extending the life of the gas mixture or the time between fresh fills (assigned to the same assignee as the present application and incorporated herein by reference). No. 09 / 780,120, incorporated by reference. A general description of the narrowing features of embodiments of laser systems, particularly those used in photolithography applications, is provided here, followed by high spectral purity or high levels of bandwidth (eg, Description of the present application by way of reference to illustrate the variations and features used within the preferred embodiment of the present application to provide an output beam having an output beam (less than 1 pm and preferably 0.6 pm or less). The patents and patent applications incorporated by reference are listed below. Such an exemplary embodiment may be used to select only the primary emission line λ _1, may be used to provide emission line selection and provide additional linewidth reduction, and Include optics for bright line selection and additional optics for narrowing the selected bright line, which may be provided by controlling (ie, reducing) the total pressure. (US patent application Ser. No. 60/50, assigned to the same assignee as the present application and incorporated herein by reference.
212, 301). Exemplary line narrowing optics included in optical module 110 include a beam expander, 09/07/07, which is incorporated herein by reference.
Any interferometer device such as an etalon or other as described in the '15, 803 application may be included, and a diffraction grating and, alternatively, one or more dispersive prisms may be used. In the case of narrow band lasers, such as those used in refractive or birefringent optical lithographic imaging systems, diffraction gratings produce a relatively higher degree of dispersion than prisms, but generally a dispersion prism (one or more). ) Shows somewhat lower efficiency. As noted above, the front optics modules are each assigned to the same assignee and are numbered 09 / 715,80, which are incorporated herein by reference.
No. 3, 09/738, 849, and 09/71
Linewidth narrowing optics as described in any of the 8,809 patent applications may be included.

Instead of having a retro-reflective grating in the rear optics module 110, the grating is replaced by a high-reflection mirror and a lower degree of dispersion is created by the dispersing prism, or alternatively. In some cases, line narrowing or bright line selection may not be performed in the rear optical module 110. When using an all-reflective imaging system,
Lasers are characterized by their characteristic broad bandwidth.
Depending on the broadband bandwidth, it may be configured for semi-narrowband operation with an output beam line width in excess of 0.6 pm, in which case the total pressure in the laser tube or by optics may be reduced. The additional narrowing of the selected bright line provided by any of the reductions would not be used.

The beam expander of the exemplary line narrowing optics of the optical module 110 preferably includes one or more prisms. The beam expander may include other beam expansion optics such as a lens assembly or a converging / diverging lens pair. The diffraction grating or high reflectivity mirror is preferably rotatable, so that the wavelengths reflected within the acceptance angle of the resonator can be selected or tuned. Alternatively, the diffraction grating, or other optical device (s), or the entire line narrowing module, may each be assigned to the same assignee as the present application and are incorporated by reference herein in their entirety. No./771,366 Patent Application and No. 6,154,4
As described in the '70 patent, it may be pressure tuned. The diffraction grating is used to disperse the beam to achieve a narrow bandwidth, and preferably retroreflects the beam in the direction of the laser tube.
lecting) for both purposes. Alternatively, a high reflectivity mirror is placed after the grating to receive the reflections from the grating and provide a Littman configuration.
The beam is reflected back in the direction of the diffraction grating of the configuration) or the diffraction grating is a transmissive diffraction grating.
on grating). One or more dispersing prisms may be used, and more than one etalon or other interfering device may be used.

One or more apertures may be included in the resonator to block stray light and match the divergence of the resonator (see the '277 patent application). As mentioned above, the front optics module may include line narrowing optics (09 / 715,803, each assigned to the same assignee as the present application and incorporated herein by reference). Nos. 09 / 738,849 and 09 / 718,809), including or in addition to an outcoupler element.

Depending on the type and degree of line narrowing and / or the desired selection and tuning, and the particular laser in which the line narrowing optics is to be installed, the following are specific with respect to FIGS. Many alternative optical device configurations can be used other than those described in. For this purpose, U.S. Patent Nos. 4,399,540, 4,905,243, 5,226,050, 5,559,816, 5,659,419, 5 No. 6,663,973, No. 5,761,236, No. 6,081,542, No. 6,061,382, No. 6,154,470, and No. 5,946,337, No. 5, Nos. 095,492, 5,684,822, 5,835,520, 5,852,627, 5,856,991, 5,898,725, 5,901, Nos. 163, 5,917,849, 5,970,082, 5,404,366, 4,975,919, 5,142,543 and 5,596,596. No. 5,802,094, No. 4,856,018, No. 5,970 082, No. 5,978,409, No. 5,999,318, No. 5,150,370 and No. 4,829,536, and German Patent DE298
No. 22,090.3, and any of the patent applications mentioned above and below herein, may be referred to to obtain line narrowing configurations that may be used in the preferred laser systems herein. Are each incorporated by reference into the description of the present application.

As already discussed, preferably there is no line narrowing optics in the susceptible or lossy resonator, instead of a single bright line (ie, Only the optics for selecting λ ₁ ) are used. However, line narrowing optics are used for further line narrowing in combination with line narrowing and / or bandwidth adjustment performed by adjusting / reducing the total pressure in the laser chamber. May be done. For example, the natural bandwidth can be used to reduce the partial pressure of the buffer gas from 1000 to 1500.
It may be adjusted to 0.5 pm by lowering to mbar. The bandwidth may then be reduced to 0.2 pm or less using linewidth optics either inside the cavity or outside the cavity. Therefore, a general description of line narrowing optics that can be used is provided in this application. The exemplary line narrowing optics is housed in an optical module 110, a rear optics module, and a beam expander for narrow band lasers such as those used in refractive or catadioptric photolithography imaging systems. , Optional etalons and gratings, which produce a relatively high degree of dispersion. Narrow linewidth packages include a beam expander and one or more etalons, which may be followed by an HR mirror as a resonant reflector. Optical Materials In all of the above and below embodiments, the material used for any dispersing prism, any beam expander prism, etalon, laser window and output coupler is preferably a molecular It is highly transmissive at wavelengths less than 200 nm, such as the 157 nm output emission wavelength of a fluorine laser. The material can also withstand long-term exposure to ultraviolet light with minimal degradation. Examples of such materials include CaF ₂ , MgF ₂ , Ba
There are F ₂ , LiF and SrF ₂ , and in some cases fluorine-doped quartz may be used. Also, in all embodiments, many optical surfaces, especially prism optical surfaces, may have anti-reflective coatings on one or more optical surfaces to minimize reflection losses and extend their lifespan; It may not have it. Power Amplifier For example, a power amplifier may follow the line narrowing oscillator described above to increase the power of the beam output by the oscillator. Preferred features of the oscillator-amplifier configuration are described in U.S. patent application Ser. No. 09/59, assigned to the same assignee and incorporated herein by reference.
No. 9,130 and No. 60 / 228,184. The amplifier is the same or separate from the discharge chamber 102. Optical or electrical delays may be used to time the discharge at the amplifier with the arrival of light pulses from the oscillator to the amplifier. Particularly with regard to the present invention, the molecular fluorine laser oscillator, maximum at lambda _1, lambda ₂
And has an advantageous output coupler with minimal transmission interference, as described in further detail below. The 157 nm beam is output from the output coupler and is incident on the amplifier of this embodiment to increase the power of the beam. That is, a high power (at least a few watts to more than 10 watts) and very narrow bandwidth beam with a highly suppressed secondary emission line λ ₂ is achieved.

With respect to a particularly preferred embodiment, a molecular fluorine laser oscillator can be used without the optics commonly used to achieve very narrow bandwidths, eg, less than 0.5 pm.
_It has an advantageous gas total pressure, or gas composition, that produces _one very narrow band of emissions. The 157 nm beam is output from the output coupler and is incident on the amplifier of this embodiment to increase the power of the beam. That is, very narrow bandwidth beams (eg, less than 0.5 pm) are achieved without high power (at least over a few watts to more than 10 watts) and without advanced, very narrow bandwidth line narrowing optics. Is done.

While illustrative drawings and specific embodiments of the present invention have been described and illustrated, it will be understood that the scope of the present invention is not limited to the specific embodiments discussed herein.
That is, the examples should be considered illustrative rather than restrictive, and as will be appreciated, by those skilled in the art, the scope of the invention described in the claims and equivalents thereto. Variations of these embodiments may be made without departing from

Further, the method claims (me
In the thod chaim), the operations are arranged in a selected typographical order. However, unless the specific order of the steps is specified or deemed necessary by one of ordinary skill in the art, the order is selected and arranged as such for printing convenience. , Is not intended to imply any particular order for performing the operations.

[Brief description of the drawings]

FIG. 1: about 0.6 p. With typical energy of about 10 mJ
FIG. 4 schematically illustrates a molecular fluorine laser emitting a 157 nm beam containing a selected emission line having a natural bandwidth of m ± 0.1 pm and having a gas mixture pressure greater than 2500 mbar.

FIG. 2. At a typical energy of about 1 mJ, about 0.2 p
FIG. 4 schematically illustrates a molecular fluorine laser emitting a 157 nm beam including a selected narrowed emission line having a bandwidth of less than 0.5 pm, such as m.

FIG. 3 shows a molecular fluorine laser including an oscillating-amplifier configuration that produces a 157 nm beam with a bandwidth of less than 0.5 pm, such as about 0.2 pm, at an energy of about 10 mJ. It is a figure which illustrates the outline of.

FIG. 4 shows a molecular fluorine laser including a low gas pressure seeded-amplifier configuration that produces a 157 nm beam containing selected emission lines having a natural bandwidth of less than 0.5 pm, such as about 0.2 pm, at an energy of about 10 mJ. It is a figure which illustrates the outline of.

5A is a diagram qualitatively illustrating a bandwidth and a spectral purity of a beam emitted by the molecular fluorine laser of FIG. 3; (B) is a diagram qualitatively illustrating the bandwidth and spectral purity of a beam emitted by the molecular fluorine laser of FIG. 4.

FIG. 6 (a) is a molecular fluorine laser emission spectrum when the F ₂ laser operates at a reduced gas pressure of 1600 mbar and has a bandwidth of less than about 0.47 pm without deconvolution by the instrument function of the spectrometer. It is a figure which illustrates quantitatively. (B) shows that the F ₂ laser is 2500
FIG. 4 quantitatively illustrates a molecular fluorine laser emission spectrum when operating at a gas pressure of ３3000 mbar and having a bandwidth of about 1 pm.

FIG. 7 illustrates the dependence of the bandwidth of the F ₂ laser emission on the total pressure of the laser gas mixture in the laser tube, with the total of the laser gas mixture at substantially constant fluorine-buffer gas distribution. 4 shows the bandwidth reduction with decreasing pressure.

FIG. 8 is a diagram illustrating an outline of an entire molecular fluorine laser system according to a preferred embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Laser chamber 2 ... Bright line selection unit 3 ... Output coupling mirror 4 ... Solid pulsar module (discharge circuit) 5 ... High reflection mirror 7 ... Synchronization module 12 ... Line narrowing module 14 ... Amplification chamber 16 ... Discharge circuit DESCRIPTION OF SYMBOLS 18 ... Seed tube 20 ... Discharge circuit (amplifier discharge module) 22 ... Attached part of discharge circuit 20 24 ... HR (high reflection) mirror

Claims (97)

[Claims] 1. A seeded oscillator comprising: a molecular fluorine (F ₂ ) laser system comprising a laser tube having a plurality of electrodes connected to a discharge circuit, wherein the laser tube is about 157.
a portion of an optical resonator for producing a laser beam comprising a first emission line of the nm multi-emission emission line, wherein said laser tube is filled with a gas mixture comprising molecular fluorine and a buffer gas; But without additional linewidth optical components to narrow the first bright line.
The first of about 157 nm having a natural line width of less than 5 pm;
A seed oscillator that is at a pressure lower than the pressure that results in the generation of a laser emission that includes an emission line; and a power amplifier to increase the power of the beam emitted by the seed oscillator to a desired power for an application process. , Including molecular fluorine laser systems. 2. The laser system of claim 1, further comprising at least one emission line selection optical component for suppressing one or more emission lines of the multi-characteristic emission line of the laser at about 157 nm. . 3. The method of claim 2, wherein only the first emission line is selected from the multiple characteristic emission lines, and any other emission line of the multiple characteristic emission lines is suppressed by the at least one emission line selection optical component. The laser system as described. 4. A molecular fluorine laser system, comprising: a discharge tube filled with a gas mixture comprising molecular fluorine and a buffer gas; and a discharge connected to a discharge circuit for applying a voltage to said gas mixture. A plurality of electrodes in a chamber, a resonator for producing a laser beam including a first emission line of a multi-characteristic emission line of about 157 nm, and a power of the beam generated by the resonator.
A power amplifier that increases to a desired power for an application process, wherein the resonator further narrows the linewidth of the first bright line. Molecular fluorine laser system, without any components, wherein the gas mixture has a sufficiently low total pressure such that the laser beam includes the first emission line of about 157 nm having a linewidth of less than 0.5 pm. 5. The laser of claim 4, wherein the resonator includes at least one line selection optical component that suppresses one or more of the approximately 157 nm multi-characteristic emission lines of the laser system. ·system. 6. The method of claim 5, wherein only the first emission line is selected from the multiple characteristic emission lines, and any other emission line of the multiple characteristic emission lines is suppressed by the at least one emission line selection optical component. Laser system. 7. The gas mixture pressure is 2000 mbar.
5. The laser system according to claim 1 or 4, wherein 8. The laser system according to claim 7, wherein the desired power corresponds to an energy between 1 and 15 mJ. 9. The laser system according to claim 1, wherein the line width of the first bright line is less than 0.4 pm. 10. The pressure of the gas mixture is 1500 mba.
10. The laser system of claim 9, wherein the value is less than r. 11. The laser system according to claim 10, wherein the desired power corresponds to an energy between 1 and 15 mJ. 12. The line width of the first bright line is 0.3 pm.
5. The laser system according to claim 1 or 4, wherein 13. The gas mixture pressure is 1300 mba
13. The laser system of claim 12, wherein the value is less than r. 14. The laser system according to claim 13, wherein the desired power corresponds to an energy between 1 and 15 mJ. 15. The line width of the first bright line is 0.2 pm.
5. The laser system according to claim 1 or 4, wherein 16. The pressure of the gas mixture is 1000 mba.
16. The laser system of claim 15, wherein the value is less than r. 17. The laser system according to claim 16, wherein the desired power corresponds to an energy between 1 and 15 mJ. 18. The line width of the first bright line is 0.15p.
The laser system according to claim 1 or 4, wherein the distance is less than m. 19. The gas mixture pressure is 800 mbar
19. The laser system according to claim 18, wherein 20. The laser system according to claim 19, wherein the desired power corresponds to an energy between 1 and 15 mJ. 21. A gas processing unit coupled to the discharge vessel for flowing gas between the discharge vessel and the gas processing unit, and one or more parameters associated with the gas mixture. 5. The laser system according to claim 1, further comprising a processor for controlling a gas flow between the gas processing unit and the discharge tube. 22. A molecular fluorine laser system, comprising: a discharge tube filled with a gas mixture comprising molecular fluorine and a buffer gas; and a discharge connected to a discharge circuit for applying a voltage to said gas mixture. A plurality of electrodes in a chamber, a resonator for producing a laser beam including a first emission line of a multi-characteristic emission line of about 157 nm, and a power of the beam generated by the resonator.
A power amplifier that increases to a desired power for an application process, wherein the resonator has at least one narrow linewidth that narrows a linewidth of the first bright line of about 157 nm. The gas mixture may further include a first bright line having a reduced line width.
A molecular fluorine laser system having a sufficiently low total pressure to have a line width of less than 2 pm. 23. The resonator of claim 22, wherein the resonator includes at least one line selection optical component for suppressing one or more of the approximately 157 nm multi-characteristic emission lines of the laser system. Laser system. 24. The method of claim 23, wherein only the first emission line is selected from the multiple emission lines and any other emission line of the multiple emission lines is suppressed by the at least one emission line selection optical component. Laser system. 25. The gas mixture pressure is 2000 mba.
23. The laser system according to claim 22, which is less than r. 26. The laser system according to claim 25, wherein the desired power corresponds to an energy between 1 and 15 mJ. 27. The line width of the first bright line is 0.15p.
23. The laser system of claim 22, wherein the distance is less than m. 28. The line width of the first bright line is 0.10 p.
23. The laser system of claim 22, wherein the distance is less than m. 29. The pressure of the gas mixture is 1500 mba
29. Any one of claims 22, 27 or 28 that is less than r
A laser system according to item 4. 30. The laser system according to claim 29, wherein the desired power corresponds to an energy between 1 and 15 mJ. 31. The pressure of the gas mixture is 1000 mba.
29. Any one of claims 22, 27 or 28 that is less than r
A laser system according to item 4. 32. The laser system according to claim 31, wherein the desired power corresponds to an energy between 1 and 15 mJ. 33. A gas processing unit coupled to the discharge tube for flowing gas between the discharge tube and the gas processing unit; and controlling one or more parameters associated with the gas mixture. 23. The laser system of claim 22, including a processor for controlling the flow of gas between the gas processing unit and the discharge tube for the purpose. 34. A molecular fluorine laser system, comprising: a discharge tube filled with a gas mixture comprising molecular fluorine and a buffer gas; and a discharge connected to a discharge circuit for applying a voltage to said gas mixture. A plurality of electrodes in a chamber; a resonator for generating a laser beam including a first emission line of a multi-characteristic emission line of about 157 nm; and a discharge tube for flowing gas between the discharge tube and a gas processing unit. A gas processing unit coupled; a processor for controlling a gas flow between the gas processing unit and the discharge vessel to control one or more parameters associated with the gas mixture; and a resonator. A power amplifier for increasing the power of the beam generated to a desired power for an application process. In the system, the gas mixture, the molecular fluorine laser system having a sufficiently low total pressure, such as including the first emission line of approximately 157nm that said laser beam has a linewidth of less than 0.5 pm. 35. The laser of claim 34, wherein the resonator includes at least one line selection optical component that suppresses one or more of the approximately 157 nm multi-characteristic emission lines of the laser system. ·system. 36. The method of claim 35, wherein only the first emission line is selected from the multiple characteristic emission lines, and any other emission line of the multiple characteristic emission lines is suppressed by the at least one emission line selection optical component. Laser system. 37. The laser system of claim 34, wherein the resonator does not include an additional line narrowing optical component that further narrows the line width of the first bright line. 38. The laser of claim 34, wherein the resonator includes at least one additional line narrowing optical component for further narrowing the line width of the first bright line.
system. 39. The pressure of the gas mixture is 2000 mba.
35. The laser system according to claim 34, which is less than r. 40. The laser system according to claim 39, wherein the desired power corresponds to an energy between 1 and 15 mJ. 41. The laser system according to claim 34, wherein said line width is less than 0.4 pm. 42. The pressure of the gas mixture is 1500 mba.
42. The laser system according to claim 41, wherein r is less than r. 43. The laser system according to claim 42, wherein said desired power corresponds to an energy between 1 and 15 mJ. 44. A molecular fluorine laser system, comprising: a discharge tube filled with a gas mixture comprising molecular fluorine and a buffer gas; and a discharge connected to a discharge circuit for applying a voltage to said gas mixture. A plurality of electrodes in a chamber; a resonator for producing a laser beam including a first emission line of a multi-characteristic emission emission line of about 157 nm; A molecule comprising: a combined gas processing unit; and a processor for controlling a flow of the gas between the gas processing unit and the discharge vessel to control one or more parameters associated with the gas mixture. In a fluorine laser system, the gas mixture includes the first bright line of about 157 nm, wherein the laser beam has a line width of less than 0.5 pm. A molecular fluorine laser system with a sufficiently low total pressure. 45. The resonator of claim 44, wherein the resonator includes at least one line selection optical component for suppressing one or more of the approximately 157 nm multi-characteristic emission lines of the laser system. Laser system. 46. The method of claim 45, wherein only the first emission line is selected from the multiple emission lines and any other emission line of the multiple emission lines is suppressed by the at least one emission line selection optical component. Laser system. 47. The laser system of claim 44, wherein the resonator does not include additional line narrowing optics to further narrow the line width of the first bright line. 48. The laser system of claim 44, wherein the resonator includes at least one additional line narrowing optical component for further narrowing the line width of the first bright line. . 49. The gas mixture having a pressure of 2000 mba
47. The laser system of claim 44, wherein said laser system is less than r. 50. The line width of the first bright line is 0.4 pm
46. The laser system of claim 44, wherein 51. The pressure of the gas mixture is 1500 mba
51. The laser system according to claim 50, wherein r is less than r. 52. The laser of any one of claims 44 to 51, further comprising a power amplifier for increasing the power of the beam generated by the resonator to a desired power for an application process. system. 53. A method for controlling the bandwidth of a molecular fluorine laser system, comprising: operating the laser system; monitoring the bandwidth of an output beam of the laser system; The beam has a line width of less than 0.5 pm
Controlling the pressure of the gas mixture in the laser tube of the laser system to a predetermined low enough pressure to include a first one of the 7 nm multi-characteristic emission lines; Power,
Amplifying the output beam to increase to a desired power for an application process, and a method of controlling the bandwidth of a molecular fluorine laser system. 54. The method of claim 1, further comprising the step of:
54. The method of claim 53, comprising the step of suppressing one or more of the multiple characteristic emission lines at 57 nm. 55. The method as recited in claim 55, wherein the suppressing step is performed at about 157 nm.
54. The method of claim 53, comprising suppressing all but the first emission line of the multi-characteristic emission line of. 56. The method of claim 53, wherein the method does not include using an additional line narrowing optical component to further narrow the line width of the first bright line. 57. The method of claim 53, further comprising the step of further narrowing the line width of the first bright line using an additional line narrowing optical component. 58. The method of claim 53, wherein said controlling the gas mixture pressure comprises controlling said pressure to less than 2000 mbar. 59. The line width of the first bright line is 0.4 pm
54. The method of claim 53 that is less than. 60. The pressure of the gas mixture is 1500 mb
60. The method according to claim 59, which is less than ar. 61. A method according to any one of claims 53, 58 or 60, wherein the desired power corresponds to an energy between 1 and 15 mJ. 62. A method for controlling the bandwidth of a molecular fluorine laser system, comprising: operating the laser system; monitoring the bandwidth of an output beam of the laser system; The beam has a line width of less than 0.5 pm
Controlling the pressure of the gas mixture in the laser tube of the laser system to a predetermined low enough pressure to include a first one of the 7 nm multi-emission emission lines. How to control. 63. The method of claim 62, further comprising amplifying the output beam to increase the power of the beam generated by a resonator to a desired power for an application process. 64. The method of claim 63, wherein said desired power corresponds to an energy between 1 and 15 mJ. 65. The method of claim 1, further comprising:
63. The method of claim 62, comprising suppressing one or more of the 57nm multi-characteristic emission lines. 66. The method of claim 62, wherein the method does not include the step of further narrowing the line width of the first bright line using an additional line narrowing optical component. 67. The method of claim 62, further comprising using an additional line narrowing optical component to further narrow the line width of the first bright line. 68. The pressure of the gas mixture is 1500 mb
63. The method of claim 62, which is less than ar. 69. A method of controlling the bandwidth of a molecular fluorine laser system, comprising the steps of operating the laser system; and wherein the output beam has a multi-characteristic emission line of about 157 nm having a linewidth of less than 0.5 pm Controlling the pressure of the gas mixture in the laser tube of the laser system to a predetermined pressure sufficiently low to include the first emission line of the method. 70. The method of claim 69, further comprising monitoring the bandwidth of the output beam of the laser system. 71. The method of claim 69, further comprising amplifying the output beam to increase the power of the beam generated by a resonator to a desired power for an application process. 72. The method of claim 71, wherein the desired power corresponds to an energy between 1 and 15 mJ. 73. The method of claim 1, further comprising:
70. The method of claim 69, comprising suppressing one or more of the multiple characteristic emission lines at 57nm. 74. The method of claim 69, wherein the method does not include the step of further narrowing the line width of the first bright line using an additional line narrowing optical component. 75. The method of claim 69, further comprising the step of further narrowing the line width of the first bright line using an additional line narrowing optical component. 76. The pressure of the gas mixture is 1500 mb
70. The method of claim 69, which is less than ar. 77. A method for controlling the bandwidth of an excimer or molecular fluorine laser beam generation system including a low pressure gas lamp and an optical amplifier, the method comprising: operating the laser system; and emitting by the lamp. Monitoring the bandwidth of the applied radiation; and a gas mixture in the gas lamp of the laser system to a predetermined pressure sufficiently low that the output beam comprises an emission line having a linewidth of less than 0.5 pm. Controlling the pressure of the beam; and
Amplifying the radiation emitted by the gas lamp to increase to a desired power for the application process. A method for controlling the bandwidth of an excimer or molecular fluorine laser beam generation system. 78. The gas pressure of the lamp is 200
78. The method of claim 77, wherein the method is controlled to be less than 0 mbar. 79. The method of claim 77, wherein said line width of said bright line is less than 0.4 pm. 80. The gas pressure of the lamp is 150
80. The method of claim 79, wherein the method is controlled to be less than 0 mbar. 81. The method according to claim 77, wherein the desired power corresponds to an energy between 1 and 15 mJ.
The method described in the section. 82. An excimer or molecular fluorine (F ₂ ) laser system comprising: a seed radiation producing excimer or molecular fluorine gas filled with a gas mixture comprising at least molecular fluorine.
A seed radiation producing excimer or molecular fluorine gas lamp, wherein the gas mixture is at a pressure lower than a pressure that results in producing a seed radiation emission having a natural linewidth of less than 0.5 pm; An excimer or molecular fluorine laser system comprising: a power amplifier for increasing the power of the radiation emitted by the product gas lamp to a desired power for the application process. 83. A molecular fluorine (F ₂ ) laser system, comprising: a seed radiation producing molecular fluorine gas lamp filled with a gas mixture comprising at least molecular fluorine, wherein the gas mixture is less than 0.5 pm. A seed radiation producing molecular fluorine gas lamp having a pressure lower than a pressure that results in producing a seed radiation emission of about 157 nm comprising a bright line having a natural line width of: And a power amplifier to increase power. 84. The pressure of the gas mixture is 2000 mb
84. The laser of claim 82 or 83, wherein
system. 85. The laser system of claim 84, wherein said desired power corresponds to an energy between 1 and 15 mJ. 86. The laser system according to claim 82, wherein the line width of the bright line is less than 0.4 pm. 87. The pressure of the gas mixture is 1500 mb
87. The laser system of claim 86, wherein said laser system is less than ar. 88. The laser system of claim 87, wherein the desired power corresponds to an energy between 1 and 15 mJ. 89. The laser system according to claim 82, wherein the line width of the bright line is less than 0.3 pm. 90. The pressure of the gas mixture is 1300 mb
90. The laser system of claim 89, wherein said laser system is less than ar. 91. The laser system of claim 90, wherein said desired power corresponds to an energy between 1 and 15 mJ. 92. The laser system according to claim 82, wherein the line width of the bright line is less than 0.2 pm. 93. The pressure of the gas mixture is 1000 mb
93. The laser system of claim 92, wherein said laser system is less than ar. 94. The laser system of claim 93, wherein said desired power corresponds to an energy between 1 and 15 mJ. 95. The line width of the first bright line is 0.15p.
84. The laser system of claim 82 or 83, wherein the distance is less than m. 96. The pressure of the gas mixture is 800 mba
97. The laser system of claim 95, wherein said laser system is less than r. 97. The laser system of claim 96, wherein said desired power corresponds to an energy between 1 and 15 mJ.